Light passing through a thin crystal can change color and the direction of its oscillations. In a molybdenum disulfide flake just hundreds of atoms thick, an infrared beam turns green, and its polarization, the way the light wave oscillates, obeys the crystal's geometry.
Physicists measured how the polarization of the doubled light depends on flake thickness, rotation of the crystal lattice, and path length. It turns out that thickness and crystal orientation set the polarization rigidly, while path length allows fine-tuning.
This is like tuning a musical instrument: the body shape sets the fundamental tone, and string tension lets you fine-tune the sound. Here, the crystal sets the baseline polarization, and the waveguide length provides precise adjustment.
Such light control is needed for optical chips where information is carried by photons. Tunable light sources and quantum circuits could become more compact and flexible.
Molybdenum disulfide is a layered material: it can be split into flakes just one atom thick, much like graphite into graphene.